The sites and nature of retinal adaptation will be studied in a defined microcircuit of the Xenopus retina, using intracellular and extracellular recording methods. Absolute dark thresholds, increment threshold functions and receptive field organization of the retinal neurons will be defined using various patterns of spatial and chromatic stimulation. The correlation between synaptic form and function will be sought by comparing synaptic ultrastructure of photoreceptor-bipolar contacts with the morphology of the bipolar cell and its receptive field organization.